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Optimization method for blue Sr2MgSi2O7:Eu2+, Dy3+ phosphors produced by microwave synthesis route
Journal of Alloys and Compounds 737 (2018) 39e45
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Optimization method for blue Sr2MgSi2O7:Eu2þ, Dy3þ phosphors produced by microwave synthesis route Liupeng Pan, Shunli Liu, Xiaolu Zhang, Olayinka Oderinde, Fang Yao**, Guodong Fu* School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing, Jiangsu Province, 211189, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2017 Received in revised form 10 November 2017 Accepted 28 November 2017 Available online 2 December 2017
We report a kind of dopant (Eu2þ) and co-dopant (Dy3þ) co-doped strontium magnesium silicate phosphor synthesized via microwave irradiation in a weak reductive atmosphere of active carbon. X-ray diffraction (XRD) patterns illustrated that the samples were nearly pure Sr2MgSi2O7 phase, in which the Sr2MgSi2O7 host phase has the maximum fraction of tetragonal crystallography Sr2MgSi2O7 phase. A series of long afterglow luminescent materials were synthesized to investigate the best synthetic conditions. The results showed that the optimal concentration of H3BO3 and the best sintering time were 5 wt% and 30 min, respectively. The phosphors made from the microwave irradiation method demonstrated excellent luminescence performance and the microwave irradiation method provide a strategy for the synthesis of other alkali earth aluminate and silicate, or other luminescent materials. © 2017 Elsevier B.V. All rights reserved.
Keywords: Sr2MgSi2O7:Eu2þ, Dy3þ Microwave Optimization Photoluminescence Long afterglow
1. Introduction Long afterglow phosphor is one kind of photoluminescent materials [1,2]. As a kind of “green light source material”, people pay greater attention towards it in the recent shortage of energy. Owing to it consumes its own stored energy in the long afterglow process, it does not need external energy supply, which makes the material irreplaceable in some specific environments. The material can be widely used in traffic safety signs [3,4], sensor and detector [5e8], biomedical imaging [9,10], photocatalysis [11,12], solar cell [13,14] and in many other fields. Rare earth doped silicate long afterglow materials have superior performance in the afterglow [15]. In addition to making up for the shortage of the aluminate long afterglow materials of poor humidity resistance, which broaden the use of long afterglow materials, it can be utilized in waterborne coatings and other moist environment directly [16,17]. Furthermore, the luminescent materials based on silicate has good chemical and thermal stability, with the sintering temperature lower than aluminates by about 100 C, while the high-purity silica raw material is cheap and easy to obtain. The silicate long afterglow materials make up for the single
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (G. Fu). https://doi.org/10.1016/j.jallcom.2017.11.343 0925-8388/© 2017 Elsevier B.V. All rights reserved.
issue of luminous color. In the early stage, the research results of doped silicate phosphors have laid a solid foundation for the study of long afterglow silicate materials [18e21]. The successful development of silicate long afterglow materials provides a valuable model for further exploration of new long afterglow materials. The synthesis method for long afterglow materials plays a significant role in determining the features of microstructure, afterglow properties, fluorescent quantum efficiency and the distribution of defects [1]. The conventional method is solid-state reaction [22], which require a high sintering temperature in order to obtain designed compositions, but a high sintering temperature can lead to agglomerated products with irregular morphologies. The chemical synthesis-liquid phase reaction such as sol-gel method [23], hydrothermal method [24], co-precipitation [25] and combustion methods [26], do not require high sintering temperature, however, the process is unfriendly to environment with the products possessing unsatisfactory long afterglow properties. Like other visible light, microwaves are polarized waves and coherent waves, following the laws of light, whose interaction with matter can be transmitted, absorbed, or reflected, depending on the nature of matter. Different interactions have been involved during the microwave process, including extraction [27], sterilization [28], homogeneous precipitation and co-precipitation [29], hydrothermal method [30], sintering method [31] and solid phase synthesis [32]. Compared with the conventional synthesis method, microwave heating has the advantages of fast heating, uniform heating,
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energy saving, high efficiency and easily control. Herein, we propose a one-step facile preparation of Eu2þ and 3þ Dy co-doped long afterglow luminescent material (Sr2MgSi2O7:Eu2þ, Dy3þ) with excellent fluorescent properties via microwave irradiation. As illustrated in Scheme 1, the Sr2MgSi2O7:Eu2þ, Dy3þ long afterglow luminescent material (SLPLM) was synthesized in the presence of carbon powders under weak reducing atmosphere, where carbon powders acted as a wave absorbing agent and thermal reductant. In addition, a layer of mullite powder with high thermal resistance was also added outside the furnace to provide a good environment for the holding crucible, and to effectively improve the microwave heating temperature, reduce the heating time, and eliminate the damage of high temperature on the microwave furnace. The obtained SLPLM phosphor exhibited well-defined cuboid-like structures and bright blue-light emission properties as well as remarkable persistent luminescence performance. 2. Experimental section 2.1. Materials Strontium carbonate (Sr(CO)3) was purchased from Sinopharm Chemical Reagents Co. Ltd. (Beijing, China). Silicon dioxide (SiO2), europium oxide (Eu2O3), dysprosium oxide (Dy2O3), boric acid (H3BO3) and carbon powders were bought from Aladdin Industrial Corporation (Shanghai, China). Basic magnesium carbonate (4MgCO3$Mg(OH)2$5H2O) was purchased from Macklin Biochemical Co. Ltd (Shanghai, China). Without otherwise stated, all other reagents and solvents were purchased from commercial suppliers and used as received without further purification. 2.2. Synthesis of Sr1.975MgSi2O7:Eu2þ0.01, Dy3þ
0.015
powders
Polycrystalline powdered samples with the nominal composition Sr1.975MgSi2O7:Eu2þ0.01, Dy3þ0.015 was prepared via a one-step facile microwave radiation reaction. Firstly, according to the molar ratio of 2:0.2:1.975:0.005:0.075, SiO2, 4MgCO3$Mg(OH)2$5H2O, Sr(CO)3, Eu2O3 and Dy2O3 were accurately weighed, respectively and mixed properly. Then H3BO3 (the ratio of H3BO3 to Sr1.975MgSi2O7:Eu2þ0.01, Dy3þ 0.015 was 2 wt %~8 wt %), which is the cosolvent, was added into the above mixed material. Afterwards,
the final mixed materials with zirconia beads were put together in an agate jar and mechanically, milled intensively using a highenergy ball mill. The samples, under a reduced atmosphere provided by carbon powders, were further sintered at high fire stall of a microwave oven (P70F23P-G5(B0)) for 20e30 min, and then allowed to cool to room temperature in the furnace. The reducing atmosphere was required to ensure the complete reduction of Eu3þ to Eu2þ. Note that this slow cooling also provides the formation of a ceramic structure for these materials. A series of Sr2MgSi2O7:Eu2þ, Dy3þ long afterglow luminescent materials (SLALM) with different contents of cosolvent and different sintering time were prepared, namely SLALM-1 (5 wt % H3BO3, 30min), SLALM-2 (5 wt % H3BO3, 25min), SLALM-3 (5 wt % H3BO3, 20min), SLALM-4 (2 wt % H3BO3, 30min), and SLALM-5 (8 wt % H3BO3, 30min). 2.3. Characterization of properties FT-IR spectra were obtained using a TENSOR27 PMA 50 (Brook, Germany). The Powder XRD data were collected on a Ultima IV (kabuskiki kaisha) X-ray diffractometer with CuKa radiation (k ¼ 1.5418 Å), with a scan speed of 10 min1, scanning angle from 10 to 80 and running at 40 kV and 20 mA. The morphologies of the phosphors and films were characterized by scanning electron microscope (SEM) and the fluorescence microscopic images were taken by inverted fluorescence microscope (EVOS, America) at room temperature. The photoluminescent properties were investigated for emission and excitation spectra at room temperature with FluoroLog 3-TCSPC (Jobin Yvon Inc. USA). Afterglow properties were obtained using Screen luminance meter ST-86LA3(Beijing, China) after the samples have been fully activated under ultraviolet (UV) light for 10 min. The decay curves were determined from an average of three independent sample measurements. 3. Results and discussion 3.1. Characterizations of the long persistent phosphors FTIR spectra has been extensively used in the identification of organic and inorganic compounds. The Sr2MgSi2O7:Eu2þ, Dy3þ phosphor was analyzed by FTIR and the spectra are shown in Fig. 1. The appearance of a broad band at 3430 cm1 is related to the
Scheme 1. Schematic illustration for the preparation of SLPLM phosphor. The digital photograph of mixed raw materials before (a) and after microwave radiation (b), digital photograph of SLPLM phosphor (c) and the digital images of the SLPLM phosphor after excitation at 365 nm for 10 min viva an ultraviolet lamp with a power of 6 W (d).
L. Pan et al. / Journal of Alloys and Compounds 737 (2018) 39e45
Fig. 1. FT-IR Spectra of SLMLP-1 phosphor.
stretching vibrations of hydroxyl (eOH) group, which might be due to the presence of moisture in the environment. It can be seen from Fig. 1 that the spectrum is level and smooth in the range of 1700e1900 cm1, which belong to the asymmetric stretching of CO2 3 ions [33], proved that the product obtained by microwave method is very pure and no raw materials [SrCO3 and 4MgCO3$Mg(OH)2$5H2O] remained. The bending of the sharp peak at 1620 cm1 is assigned to the Mg2þ while the vibration bands in the region of 1320-1410 cm1 are assigned to the bending of Sr2þ [34]. According to previous studies on silicate materials, the absorption bands of silicate groups were clearly evident [35]. In the spectrum, the peaks at 1010 cm1 and 965 cm1 are assigned to the SieOeSi asymmetric stretch while the band at 924 cm1 is assigned to the SieO symmetric stretch. In the fingerprint region, the sharp peak at 840 cm1 is allotted to the Sr2þ ions [36], while the SieOeSi vibrational mode is represented by the bands at 670, 624 and 570 cm1, respectively with the absorption peak at 485 cm1 attributed to the MgeO modes [37]. Hence, we obtained pure phase long afterglow phosphors via microwave heating method. In order to further investigate the purity of the phosphor and determine the crystal structure, a series of the powder XRD analysis was been carried out. XRD patterns of the long afterglow luminescent materials with different contents of cosolvent and different sintering time and the standard atlas of Sr2MgSi2O7 (PDF 75-1736 [34]) are shown in Fig. 2. Due to the same composition, it is evident to find that the peak positions were almost the same for the five samples and indicating that all the samples showed high degree of crystallinity and the main crystalline phase was Sr2MgSi2O7 with the crystals belonging to tetragonal system. Compared to the standard card, the XRD pattern of SLPLM-3 has three more weak peaks at 22.74 , 31.96 and 32.86 while LPLM-4 have sharp peaks appearing at 22.72 , 32 , 32.82 , 38.88 and 40.34 of 2q, which were the characteristic diffraction peaks of Sr3MgSi2O8 [PDF 100075] [33,35]. This depicted that the product obtained will contain some impure phases when the sintering time is not long enough or that the solvent is insufficient. Furthermore, since samples SLPLM1, SLPLM-2, SLPLM-5 are almost identical to Sr2MgSi2O7, proved that the crystallization of the samples can be improved by changing the sintering system and method. The morphological structure of long afterglow luminescent material synthesized via microwave sintering was studied using
41
Fig. 2. XRD patterns of all the as-prepared phosphors and the Sr2MgSi2O7 simulated pattern (PDF 75-1736).
SEM. The information about morphology, grain size, and shape of the Sr2MgSi2O7:Eu2þ, Dy3þ phosphor can be obtained from Fig. 3(a), (b) and (c) with different magnification. The SEM images confirmed that the particles contain cuboid-like microstructures, while the surface morphology were non-uniform and randomly stacked together, which indicated that the distribution of the crystallite sizes are inhomogeneous [38,39]. As shown in Fig. 3d, strontium (Sr), magnesium (Mg), silicon (Si), and oxygen (O) in the EDX spectra of the phosphor appeared evidently. Meanwhile, the appearance of europium (Eu) and dysprosium (Dy) proved that the rare earth metals were successfully doped into the silicate matrix, which is in accordance with the results of FTIR and XRD. In order to investigate the photoluminescence properties of the long afterglow materials synthesized via different microwave hearting time, the excitation, emission spectra and the CIE Chromaticity diagram of a series of Sr2MgSi2O7:Eu2þ,Dy3þ phosphors were depicted in Fig. 4. As shown in Fig. 4 a, it can be obvious that all the three samples have a strong absorption at 300e450 nm, which demonstrates that the phosphor powder can be efficiently excited by both visible light and UV light. Fig. 4b shows the emission of three samples at different reaction times under 365 nm laser excitation. There is an observable blue light emission in the emission wavelength at around 465 nm, which belongs to the 4f7-4f65d1 transition of Eu2þ. The absence of emission spectra of Eu3þ and Dy3þ proved that Eu3þ ions have been reduced to Eu2þ completely by carbon powders, while the co-doped Dy3þ did not emit and transfer the absorbed energy to Eu2þ ions in the magnesium silicate matrixes of crystal lattice. Moreover, the intensity of the excitation and emission peaks enhance with the increase of microwave heating time. Commission International d’ Eclairage (CIE) 1931 system [35] shown in Fig. 4c, is used to evaluate the color luminescence emission intensities and also to identify the main emission wavelength of the phosphors in order to evaluate the material performance. It can be seen from the graph that the intensities of samples made from microwave sintering for 30 min and 25 min are almost the same, with a slight decrease in that of 20 min. Fig. 5 shows the excitation, emission spectra and the CIE Chromaticity diagram for Sr2MgSi2O7:Eu2þ, Dy3þ long afterglow luminescent materials with different amounts of H3BO3. It is obvious that with the amount of H3BO3 increasing from 2 wt% to 5 wt%, the
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Fig. 3. SEM images: (a, b, c) with different magnifications and EDX spectra (d) of the SLMLP-1 phosphor.
Fig. 4. (a) Excitation, (b) emission spectra for Emission spectra were collected at l ¼ 365 nm, and the excitation spectra were collected at the l max of emission spectrum for each sample. (c) CIE Chromaticity diagram of Sr2MgSi2O7:Eu2þ, Dy3þ phosphors made from different microwave hearting time.
intensity of spectral peak increased significantly. However, upon further increasing the concentration of H3BO3 to 8 wt%, there was a slight decrease in intensity. Moreover, the color luminescence emission intensities can also be visually observed from Fig. 5c. The result showed that the sample with 5 wt% H3BO3 exhibited high brightness and best color luminescence emission intensities.
In order to further investigate the photoluminescence properties of the phosphors, the decay curves of the long afterglow of the samples of SLPLM-1, SLPLM-2 and SLPLM-3 were determined after excitation at 365 nm for 10 min using a 6 W ultraviolet lamp, as shown in Fig. 6a. The decay times of the phosphors can be calculated by a curve fitting technique [40,41], with all the three samples
L. Pan et al. / Journal of Alloys and Compounds 737 (2018) 39e45
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Fig. 5. (a) Excitation, (b) emission spectra collected at l ¼ 365 nm, and the excitation spectra collected at the l max of emission spectrum for each sample. (c) CIE Chromaticity diagram of Sr2MgSi2O7:Eu2þ, Dy3þ phosphors made from different amount of cosolvent H3BO3.
Fig. 6. (a) The afterglow decay curves of phosphors prepared at different microwave sintering times. (b) Images acquired with a smartphone and intensity profile scan at 1 min after stopping the excitation.
undergone decay according to quadratic exponential functions expressed in the following form:
It ¼ A1 expð t=t1 Þ þ A2 expð t=t2 Þ
(1)
where, It is the phosphorescence intensity at time t, A1and A2 are constants, t is time, t1 and t2 are decay constants, which decide the time needed when the afterglow at fast, slow decay phase decays to 1/e of initial intensity, respectively [42]. Decay curves successfully fit into equation (1) and the fitting results of the parameters, t1 and t2 shown in Table 1. Obviously, all three samples can phosphoresce continuously for more than 25 min, while the afterglow decay curves consist of two stages, the initial rapid decaying process and the subsequent slow decaying process. The duration of the afterglow depends mainly on the time of slow decay process, therefore it can be well reacted by t2 [41]. Comparing the value of t2 in Table 1, it is found that SLPLM-1 shows slower light decay and has the highest initial brightness, which is consistent with the results of
Table 1 The fitting results of t1 and t2 of phosphors. Phosphors
t1 (s)
t2 (s)
SLPLM-1 SLPLM-2 SLPLM-3 SLPLM-4 SLPLM-5
35.523 32.755 26.708 34.406 32.161
480.659 414.586 391.234 393.536 472.750
the emission spectrum. Furthermore, the decay curves of SLPLM-2 and SLPLM-3 are almost coincident in Fig. 6a, however, via carefully comparing the data, it is found that the phosphorescence intensity of SLPLM-2 is weaker while the afterglow property is a little lower than the SLPLM-3, which demonstrates that the duration of afterglow is not directly related to the initial brightness of phosphor. This can be further proven by the images and color intensity profile scans, as depicted in Fig. 6b.
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Fig. 7. (a) The afterglow decay curves of phosphors prepared with different amounts of H3BO3. (b) Images acquired with a smartphone and intensity profile scan at 1 min after stop the excitation.
For phosphors prepared with different amounts of H3BO3, the fitted attenuation curves and image with intensity profile scan spectra were depicted in Fig. 7a and b, respectively. It can be seen that the phosphorescence intensity of the samples gradually decreases with increase in time, which is best fitted by quadratic exponential functions, with the parameters being illustrated in Table 1. When the concentration of H3BO3 is reduced to 2 wt% from 5 wt%, the persistent luminescence property became markedly low, while the t2 of SLPLM-5 (472.750s) is very close with the SLPLM-1 (480.659s), which means that the afterglow shown by SLPLM-5 is almost the same with SLPLM-1, which is in agreement with the emission spectra. In all, the results revealed that 5 wt % was the optimal concentration for H3BO3, while the best sintering time was 30 min. 4. Conclusion In summary, a series of Sr2MgSi2O7:Eu2þ, Dy3þ long afterglow luminescent materials were successfully prepared via simply microwave radiation method. The particles depicted a cuboid-like microstructure while the chemical composition of the sintered Sr2MgSi2O7:Eu2þ, Dy3þ phosphors was confirmed by EDX. The results of FTIR and XRD revealed that all the phosphors show the same components and the crystal structure is consistent with standard tetragonal crystallography as Sr2MgSi2O7. CIE color chromaticity diagram and intensity profile scan confirmed that the prepared Sr2MgSi2O7:Eu2þ, Dy3þ phosphors would emit bright blue color, which indicated that SLPLM is emitted from the same center of Eu2þ ions. Furthermore, it is found that the addition of H3BO3 is beneficial to the formation of Sr2MgSi2O7: Eu2þ, Dy3þ and that appropriate amount of H3BO3 addition helps to improve the afterglow performance of samples. The optical properties of the samples can also be improved by changing the sintering time and method. Acknowledgements This work was supported by National Natural Science Foundation of China under the Grant 21274020 and 21304019. References [1] Y. Li, M. Gecevicius, J.R. Qiu, Long persistent phosphors-from fundamentals to applications, Chem. Soc. Rev. 45 (2016) 2090e2136. [2] K.V.D. Eeckhout, P.F. Smet, D. Poelman, Persistent luminescence in Eu2þdoped compounds: a review, Materials 3 (2010) 2536e2566. [3] S.Y. Kaya, E. Karacaoglu, B. Karasu, Effect of Al/Sr ratio on the luminescence properties of SrAl2O4: Eu2þ, Dy3þ phosphors, Ceram. Int. 38 (2012) 3701e3706.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Optimization method for blue Sr2MgSi2O7:Eu2þ, Dy3þ phosphors produced by microwave synthesis route Liupeng Pan, Shunli Liu, Xiaolu Zhang, Olayinka Oderinde, Fang Yao**, Guodong Fu* School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing, Jiangsu Province, 211189, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2017 Received in revised form 10 November 2017 Accepted 28 November 2017 Available online 2 December 2017
We report a kind of dopant (Eu2þ) and co-dopant (Dy3þ) co-doped strontium magnesium silicate phosphor synthesized via microwave irradiation in a weak reductive atmosphere of active carbon. X-ray diffraction (XRD) patterns illustrated that the samples were nearly pure Sr2MgSi2O7 phase, in which the Sr2MgSi2O7 host phase has the maximum fraction of tetragonal crystallography Sr2MgSi2O7 phase. A series of long afterglow luminescent materials were synthesized to investigate the best synthetic conditions. The results showed that the optimal concentration of H3BO3 and the best sintering time were 5 wt% and 30 min, respectively. The phosphors made from the microwave irradiation method demonstrated excellent luminescence performance and the microwave irradiation method provide a strategy for the synthesis of other alkali earth aluminate and silicate, or other luminescent materials. © 2017 Elsevier B.V. All rights reserved.
Keywords: Sr2MgSi2O7:Eu2þ, Dy3þ Microwave Optimization Photoluminescence Long afterglow
1. Introduction Long afterglow phosphor is one kind of photoluminescent materials [1,2]. As a kind of “green light source material”, people pay greater attention towards it in the recent shortage of energy. Owing to it consumes its own stored energy in the long afterglow process, it does not need external energy supply, which makes the material irreplaceable in some specific environments. The material can be widely used in traffic safety signs [3,4], sensor and detector [5e8], biomedical imaging [9,10], photocatalysis [11,12], solar cell [13,14] and in many other fields. Rare earth doped silicate long afterglow materials have superior performance in the afterglow [15]. In addition to making up for the shortage of the aluminate long afterglow materials of poor humidity resistance, which broaden the use of long afterglow materials, it can be utilized in waterborne coatings and other moist environment directly [16,17]. Furthermore, the luminescent materials based on silicate has good chemical and thermal stability, with the sintering temperature lower than aluminates by about 100 C, while the high-purity silica raw material is cheap and easy to obtain. The silicate long afterglow materials make up for the single
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (G. Fu). https://doi.org/10.1016/j.jallcom.2017.11.343 0925-8388/© 2017 Elsevier B.V. All rights reserved.
issue of luminous color. In the early stage, the research results of doped silicate phosphors have laid a solid foundation for the study of long afterglow silicate materials [18e21]. The successful development of silicate long afterglow materials provides a valuable model for further exploration of new long afterglow materials. The synthesis method for long afterglow materials plays a significant role in determining the features of microstructure, afterglow properties, fluorescent quantum efficiency and the distribution of defects [1]. The conventional method is solid-state reaction [22], which require a high sintering temperature in order to obtain designed compositions, but a high sintering temperature can lead to agglomerated products with irregular morphologies. The chemical synthesis-liquid phase reaction such as sol-gel method [23], hydrothermal method [24], co-precipitation [25] and combustion methods [26], do not require high sintering temperature, however, the process is unfriendly to environment with the products possessing unsatisfactory long afterglow properties. Like other visible light, microwaves are polarized waves and coherent waves, following the laws of light, whose interaction with matter can be transmitted, absorbed, or reflected, depending on the nature of matter. Different interactions have been involved during the microwave process, including extraction [27], sterilization [28], homogeneous precipitation and co-precipitation [29], hydrothermal method [30], sintering method [31] and solid phase synthesis [32]. Compared with the conventional synthesis method, microwave heating has the advantages of fast heating, uniform heating,
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energy saving, high efficiency and easily control. Herein, we propose a one-step facile preparation of Eu2þ and 3þ Dy co-doped long afterglow luminescent material (Sr2MgSi2O7:Eu2þ, Dy3þ) with excellent fluorescent properties via microwave irradiation. As illustrated in Scheme 1, the Sr2MgSi2O7:Eu2þ, Dy3þ long afterglow luminescent material (SLPLM) was synthesized in the presence of carbon powders under weak reducing atmosphere, where carbon powders acted as a wave absorbing agent and thermal reductant. In addition, a layer of mullite powder with high thermal resistance was also added outside the furnace to provide a good environment for the holding crucible, and to effectively improve the microwave heating temperature, reduce the heating time, and eliminate the damage of high temperature on the microwave furnace. The obtained SLPLM phosphor exhibited well-defined cuboid-like structures and bright blue-light emission properties as well as remarkable persistent luminescence performance. 2. Experimental section 2.1. Materials Strontium carbonate (Sr(CO)3) was purchased from Sinopharm Chemical Reagents Co. Ltd. (Beijing, China). Silicon dioxide (SiO2), europium oxide (Eu2O3), dysprosium oxide (Dy2O3), boric acid (H3BO3) and carbon powders were bought from Aladdin Industrial Corporation (Shanghai, China). Basic magnesium carbonate (4MgCO3$Mg(OH)2$5H2O) was purchased from Macklin Biochemical Co. Ltd (Shanghai, China). Without otherwise stated, all other reagents and solvents were purchased from commercial suppliers and used as received without further purification. 2.2. Synthesis of Sr1.975MgSi2O7:Eu2þ0.01, Dy3þ
0.015
powders
Polycrystalline powdered samples with the nominal composition Sr1.975MgSi2O7:Eu2þ0.01, Dy3þ0.015 was prepared via a one-step facile microwave radiation reaction. Firstly, according to the molar ratio of 2:0.2:1.975:0.005:0.075, SiO2, 4MgCO3$Mg(OH)2$5H2O, Sr(CO)3, Eu2O3 and Dy2O3 were accurately weighed, respectively and mixed properly. Then H3BO3 (the ratio of H3BO3 to Sr1.975MgSi2O7:Eu2þ0.01, Dy3þ 0.015 was 2 wt %~8 wt %), which is the cosolvent, was added into the above mixed material. Afterwards,
the final mixed materials with zirconia beads were put together in an agate jar and mechanically, milled intensively using a highenergy ball mill. The samples, under a reduced atmosphere provided by carbon powders, were further sintered at high fire stall of a microwave oven (P70F23P-G5(B0)) for 20e30 min, and then allowed to cool to room temperature in the furnace. The reducing atmosphere was required to ensure the complete reduction of Eu3þ to Eu2þ. Note that this slow cooling also provides the formation of a ceramic structure for these materials. A series of Sr2MgSi2O7:Eu2þ, Dy3þ long afterglow luminescent materials (SLALM) with different contents of cosolvent and different sintering time were prepared, namely SLALM-1 (5 wt % H3BO3, 30min), SLALM-2 (5 wt % H3BO3, 25min), SLALM-3 (5 wt % H3BO3, 20min), SLALM-4 (2 wt % H3BO3, 30min), and SLALM-5 (8 wt % H3BO3, 30min). 2.3. Characterization of properties FT-IR spectra were obtained using a TENSOR27 PMA 50 (Brook, Germany). The Powder XRD data were collected on a Ultima IV (kabuskiki kaisha) X-ray diffractometer with CuKa radiation (k ¼ 1.5418 Å), with a scan speed of 10 min1, scanning angle from 10 to 80 and running at 40 kV and 20 mA. The morphologies of the phosphors and films were characterized by scanning electron microscope (SEM) and the fluorescence microscopic images were taken by inverted fluorescence microscope (EVOS, America) at room temperature. The photoluminescent properties were investigated for emission and excitation spectra at room temperature with FluoroLog 3-TCSPC (Jobin Yvon Inc. USA). Afterglow properties were obtained using Screen luminance meter ST-86LA3(Beijing, China) after the samples have been fully activated under ultraviolet (UV) light for 10 min. The decay curves were determined from an average of three independent sample measurements. 3. Results and discussion 3.1. Characterizations of the long persistent phosphors FTIR spectra has been extensively used in the identification of organic and inorganic compounds. The Sr2MgSi2O7:Eu2þ, Dy3þ phosphor was analyzed by FTIR and the spectra are shown in Fig. 1. The appearance of a broad band at 3430 cm1 is related to the
Scheme 1. Schematic illustration for the preparation of SLPLM phosphor. The digital photograph of mixed raw materials before (a) and after microwave radiation (b), digital photograph of SLPLM phosphor (c) and the digital images of the SLPLM phosphor after excitation at 365 nm for 10 min viva an ultraviolet lamp with a power of 6 W (d).
L. Pan et al. / Journal of Alloys and Compounds 737 (2018) 39e45
Fig. 1. FT-IR Spectra of SLMLP-1 phosphor.
stretching vibrations of hydroxyl (eOH) group, which might be due to the presence of moisture in the environment. It can be seen from Fig. 1 that the spectrum is level and smooth in the range of 1700e1900 cm1, which belong to the asymmetric stretching of CO2 3 ions [33], proved that the product obtained by microwave method is very pure and no raw materials [SrCO3 and 4MgCO3$Mg(OH)2$5H2O] remained. The bending of the sharp peak at 1620 cm1 is assigned to the Mg2þ while the vibration bands in the region of 1320-1410 cm1 are assigned to the bending of Sr2þ [34]. According to previous studies on silicate materials, the absorption bands of silicate groups were clearly evident [35]. In the spectrum, the peaks at 1010 cm1 and 965 cm1 are assigned to the SieOeSi asymmetric stretch while the band at 924 cm1 is assigned to the SieO symmetric stretch. In the fingerprint region, the sharp peak at 840 cm1 is allotted to the Sr2þ ions [36], while the SieOeSi vibrational mode is represented by the bands at 670, 624 and 570 cm1, respectively with the absorption peak at 485 cm1 attributed to the MgeO modes [37]. Hence, we obtained pure phase long afterglow phosphors via microwave heating method. In order to further investigate the purity of the phosphor and determine the crystal structure, a series of the powder XRD analysis was been carried out. XRD patterns of the long afterglow luminescent materials with different contents of cosolvent and different sintering time and the standard atlas of Sr2MgSi2O7 (PDF 75-1736 [34]) are shown in Fig. 2. Due to the same composition, it is evident to find that the peak positions were almost the same for the five samples and indicating that all the samples showed high degree of crystallinity and the main crystalline phase was Sr2MgSi2O7 with the crystals belonging to tetragonal system. Compared to the standard card, the XRD pattern of SLPLM-3 has three more weak peaks at 22.74 , 31.96 and 32.86 while LPLM-4 have sharp peaks appearing at 22.72 , 32 , 32.82 , 38.88 and 40.34 of 2q, which were the characteristic diffraction peaks of Sr3MgSi2O8 [PDF 100075] [33,35]. This depicted that the product obtained will contain some impure phases when the sintering time is not long enough or that the solvent is insufficient. Furthermore, since samples SLPLM1, SLPLM-2, SLPLM-5 are almost identical to Sr2MgSi2O7, proved that the crystallization of the samples can be improved by changing the sintering system and method. The morphological structure of long afterglow luminescent material synthesized via microwave sintering was studied using
41
Fig. 2. XRD patterns of all the as-prepared phosphors and the Sr2MgSi2O7 simulated pattern (PDF 75-1736).
SEM. The information about morphology, grain size, and shape of the Sr2MgSi2O7:Eu2þ, Dy3þ phosphor can be obtained from Fig. 3(a), (b) and (c) with different magnification. The SEM images confirmed that the particles contain cuboid-like microstructures, while the surface morphology were non-uniform and randomly stacked together, which indicated that the distribution of the crystallite sizes are inhomogeneous [38,39]. As shown in Fig. 3d, strontium (Sr), magnesium (Mg), silicon (Si), and oxygen (O) in the EDX spectra of the phosphor appeared evidently. Meanwhile, the appearance of europium (Eu) and dysprosium (Dy) proved that the rare earth metals were successfully doped into the silicate matrix, which is in accordance with the results of FTIR and XRD. In order to investigate the photoluminescence properties of the long afterglow materials synthesized via different microwave hearting time, the excitation, emission spectra and the CIE Chromaticity diagram of a series of Sr2MgSi2O7:Eu2þ,Dy3þ phosphors were depicted in Fig. 4. As shown in Fig. 4 a, it can be obvious that all the three samples have a strong absorption at 300e450 nm, which demonstrates that the phosphor powder can be efficiently excited by both visible light and UV light. Fig. 4b shows the emission of three samples at different reaction times under 365 nm laser excitation. There is an observable blue light emission in the emission wavelength at around 465 nm, which belongs to the 4f7-4f65d1 transition of Eu2þ. The absence of emission spectra of Eu3þ and Dy3þ proved that Eu3þ ions have been reduced to Eu2þ completely by carbon powders, while the co-doped Dy3þ did not emit and transfer the absorbed energy to Eu2þ ions in the magnesium silicate matrixes of crystal lattice. Moreover, the intensity of the excitation and emission peaks enhance with the increase of microwave heating time. Commission International d’ Eclairage (CIE) 1931 system [35] shown in Fig. 4c, is used to evaluate the color luminescence emission intensities and also to identify the main emission wavelength of the phosphors in order to evaluate the material performance. It can be seen from the graph that the intensities of samples made from microwave sintering for 30 min and 25 min are almost the same, with a slight decrease in that of 20 min. Fig. 5 shows the excitation, emission spectra and the CIE Chromaticity diagram for Sr2MgSi2O7:Eu2þ, Dy3þ long afterglow luminescent materials with different amounts of H3BO3. It is obvious that with the amount of H3BO3 increasing from 2 wt% to 5 wt%, the
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L. Pan et al. / Journal of Alloys and Compounds 737 (2018) 39e45
Fig. 3. SEM images: (a, b, c) with different magnifications and EDX spectra (d) of the SLMLP-1 phosphor.
Fig. 4. (a) Excitation, (b) emission spectra for Emission spectra were collected at l ¼ 365 nm, and the excitation spectra were collected at the l max of emission spectrum for each sample. (c) CIE Chromaticity diagram of Sr2MgSi2O7:Eu2þ, Dy3þ phosphors made from different microwave hearting time.
intensity of spectral peak increased significantly. However, upon further increasing the concentration of H3BO3 to 8 wt%, there was a slight decrease in intensity. Moreover, the color luminescence emission intensities can also be visually observed from Fig. 5c. The result showed that the sample with 5 wt% H3BO3 exhibited high brightness and best color luminescence emission intensities.
In order to further investigate the photoluminescence properties of the phosphors, the decay curves of the long afterglow of the samples of SLPLM-1, SLPLM-2 and SLPLM-3 were determined after excitation at 365 nm for 10 min using a 6 W ultraviolet lamp, as shown in Fig. 6a. The decay times of the phosphors can be calculated by a curve fitting technique [40,41], with all the three samples
L. Pan et al. / Journal of Alloys and Compounds 737 (2018) 39e45
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Fig. 5. (a) Excitation, (b) emission spectra collected at l ¼ 365 nm, and the excitation spectra collected at the l max of emission spectrum for each sample. (c) CIE Chromaticity diagram of Sr2MgSi2O7:Eu2þ, Dy3þ phosphors made from different amount of cosolvent H3BO3.
Fig. 6. (a) The afterglow decay curves of phosphors prepared at different microwave sintering times. (b) Images acquired with a smartphone and intensity profile scan at 1 min after stopping the excitation.
undergone decay according to quadratic exponential functions expressed in the following form:
It ¼ A1 expð t=t1 Þ þ A2 expð t=t2 Þ
(1)
where, It is the phosphorescence intensity at time t, A1and A2 are constants, t is time, t1 and t2 are decay constants, which decide the time needed when the afterglow at fast, slow decay phase decays to 1/e of initial intensity, respectively [42]. Decay curves successfully fit into equation (1) and the fitting results of the parameters, t1 and t2 shown in Table 1. Obviously, all three samples can phosphoresce continuously for more than 25 min, while the afterglow decay curves consist of two stages, the initial rapid decaying process and the subsequent slow decaying process. The duration of the afterglow depends mainly on the time of slow decay process, therefore it can be well reacted by t2 [41]. Comparing the value of t2 in Table 1, it is found that SLPLM-1 shows slower light decay and has the highest initial brightness, which is consistent with the results of
Table 1 The fitting results of t1 and t2 of phosphors. Phosphors
t1 (s)
t2 (s)
SLPLM-1 SLPLM-2 SLPLM-3 SLPLM-4 SLPLM-5
35.523 32.755 26.708 34.406 32.161
480.659 414.586 391.234 393.536 472.750
the emission spectrum. Furthermore, the decay curves of SLPLM-2 and SLPLM-3 are almost coincident in Fig. 6a, however, via carefully comparing the data, it is found that the phosphorescence intensity of SLPLM-2 is weaker while the afterglow property is a little lower than the SLPLM-3, which demonstrates that the duration of afterglow is not directly related to the initial brightness of phosphor. This can be further proven by the images and color intensity profile scans, as depicted in Fig. 6b.
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L. Pan et al. / Journal of Alloys and Compounds 737 (2018) 39e45
Fig. 7. (a) The afterglow decay curves of phosphors prepared with different amounts of H3BO3. (b) Images acquired with a smartphone and intensity profile scan at 1 min after stop the excitation.
For phosphors prepared with different amounts of H3BO3, the fitted attenuation curves and image with intensity profile scan spectra were depicted in Fig. 7a and b, respectively. It can be seen that the phosphorescence intensity of the samples gradually decreases with increase in time, which is best fitted by quadratic exponential functions, with the parameters being illustrated in Table 1. When the concentration of H3BO3 is reduced to 2 wt% from 5 wt%, the persistent luminescence property became markedly low, while the t2 of SLPLM-5 (472.750s) is very close with the SLPLM-1 (480.659s), which means that the afterglow shown by SLPLM-5 is almost the same with SLPLM-1, which is in agreement with the emission spectra. In all, the results revealed that 5 wt % was the optimal concentration for H3BO3, while the best sintering time was 30 min. 4. Conclusion In summary, a series of Sr2MgSi2O7:Eu2þ, Dy3þ long afterglow luminescent materials were successfully prepared via simply microwave radiation method. The particles depicted a cuboid-like microstructure while the chemical composition of the sintered Sr2MgSi2O7:Eu2þ, Dy3þ phosphors was confirmed by EDX. The results of FTIR and XRD revealed that all the phosphors show the same components and the crystal structure is consistent with standard tetragonal crystallography as Sr2MgSi2O7. CIE color chromaticity diagram and intensity profile scan confirmed that the prepared Sr2MgSi2O7:Eu2þ, Dy3þ phosphors would emit bright blue color, which indicated that SLPLM is emitted from the same center of Eu2þ ions. Furthermore, it is found that the addition of H3BO3 is beneficial to the formation of Sr2MgSi2O7: Eu2þ, Dy3þ and that appropriate amount of H3BO3 addition helps to improve the afterglow performance of samples. The optical properties of the samples can also be improved by changing the sintering time and method. Acknowledgements This work was supported by National Natural Science Foundation of China under the Grant 21274020 and 21304019. References [1] Y. Li, M. Gecevicius, J.R. Qiu, Long persistent phosphors-from fundamentals to applications, Chem. Soc. Rev. 45 (2016) 2090e2136. [2] K.V.D. Eeckhout, P.F. Smet, D. Poelman, Persistent luminescence in Eu2þdoped compounds: a review, Materials 3 (2010) 2536e2566. [3] S.Y. Kaya, E. Karacaoglu, B. Karasu, Effect of Al/Sr ratio on the luminescence properties of SrAl2O4: Eu2þ, Dy3þ phosphors, Ceram. Int. 38 (2012) 3701e3706.
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